Hm. "Keeping track of characters." Much depends on what you mean by keeping track. What's happening that is causing you to feel you are losing track? How does that manifest?
Some things are straightforward. Is this character human or elf or what? If there's any sort of chronology involved, when do events happen for this character? That sort of thing is readily kept. For me, it's character pages inside my Scrivener project. Names (which will slip around on me during writing), physical appearance, etc.
Some things are less straightforward. For example, this character's relations with other characters. My MC might regard Character X a certain way, but others have a different opinion and, depending the POV of the story, the reader might have yet another opinion. Moreover, some relationships will change over time--in those cases, I'll have an entry about character arc.
And some things just plain can't be captured. How a character talks. How she reacts in a certain situation or around certain people. Tics. Many of these details only develop during writing. Ideally, I make additional notes as these develop. I do not live in an ideal world.
Finally, my own notions about characters (and plot and all the rest of the wretched business) evolve over time. I have found that no matter what I write, no matter how carefully I plan, at some point I'll be looking at a character file and think "no, that's not it at all." I usually wind up writing new notes below the old ones, just in case it turns out I have yet another change of heart. It happens.
In short, I am resigned to keeping track of my characters more or less like a parent keeps track of their teenage children. We have a rough idea of what and how they're doing, who their friends are, and we more or less assume they're still in the same city. Until they aren't.
